1. Field of the Invention
Various methods exist for measuring bending of structural members. One well-known method is to measure stress on or in the members using resistive strain gauges arranged on the surface in patterns such that the bending can be inferred from a knowledge of the modulus of elasticity of the member. Under some conditions it is advantageous to measure stress or deformation using optical fibers. Advantages over electrical methods include immunity from electrical interference, light weight, ability to resist chemical attack, and operation at high temperatures. Applications include measuring and controlling vibration, sag, and deflection in aircraft wings, helicopter rotors, sailing vessel masts, large structures, machinery, boiler tubes, and robotic arms. Other applications include measuring the position of control pedals, active wing surfaces, rudders, and the like.
It is possible to embed optical fibers in composite structures so that they perform a combination of structural and sensing functions. For this purpose it is advantageous to have sensors that are unbent when at rest, so that they can lie parallel to many other straight fibers.
2. Description of the Related Art
Many types of optical fiber sensors have been developed for the measurement of stress and position. Most employ interference techniques to measure changes in length or bend radius of the fiber. Most of these techniques rely on detecting standing waves set up in the fiber by reflecting part of the light back from its distal end. These techniques are very sensitive (comparable to strain gauges) but require complex and expensive measurement techniques such as interferometry or optical time domain reflectometry (OTDR) for their execution. Measurements are very sensitive to changes in temperature, requiring elaborate compensation techniques. Another limitation of many of the interference techniques is insensitivity to direction because the measurement is made by counting the number of interference peaks due to distortion of a fiber. Thus, for example, shortening of the fiber is indistinguishable from elongation, or bending up is the same as bending down; unless the fibers are arranged in appropriate curves or other special geometric arrangements.
Equipment for performing interference measurements tends to be bulky and expensive, requiring frequent adjustment. It must be capable of distinguishing peaks at spacings of the order of 0.5 to 1 micron or less. This has limited most fiber optic stress measurements to tests which can be performed under carefully controlled laboratory conditions.
Non-interference techniques can be used to measure bending in fiber optics. It is well known that light leaks out of the core of an optical fiber if it impinges on the cladding at a sufficiently large angle with respect to the long axis of the fiber. For every fiber, there is a critical angle dependent on the indices of refraction of core and cladding, beyond which light will escape. If the fiber is bent, some of the light in the core will exceed this angle and escape. This effect has been used to build "microbending" sensors, which simply measure the percentage of transmission of light down a fiber. These suffer from relative insensitivity (little light is lost) at small angles. Usually a microbend sensor consists of a fiber placed in a corrugated fixture such that a force applied to the fixture will create many sharp bends in the fiber. Microbend sensors are used to measure pressures, forces, and displacement. These sensors also do not measure the direction of the force unless pre-tension is applied.
Other fiber optic sensors have been constructed in which the cladding is removed from the core, or the cladding and some of the core are etched away. These sensors may be more sensitive to bending than untreated fibers, but, like other bending sensors mentioned above, give no information about the direction of a bend unless they are bent at rest. They are thus unsuitable for incorporating in a simple manner in composite structures containing many parallel fibers with sensory and structural properties.
Other fiber optic sensors have been constructed which use thin films in place of the cladding, to give location information based on the wavelength of the filter produced by the thin film. This technique shows no improvement in sensitivity over other fiber optic sensing techniques, so interference techniques must be used to obtain useful outputs.